Support choosing channel to download SDK components from.

src/channel-id-mapper.ts

@@ -0,0 +1,13 @@
+export function getChannelId(channelName: string): number {
+  if (channelName === 'stable') {
+    return 0;
+  } else if (channelName === 'beta') {
+    return 1;
+  } else if (channelName === 'dev') {
+    return 2;
+  } else if (channelName === 'canary') {
+    return 3;
+  } else {
+    throw new Error(`Unexpected channel name: '${channelName}'.`);
+  }
+}